Thin film magnetoresistive random access memory (MRAM), which includes a plurality of magnetic tunnel junction (MTJ) cells, can be fabricated in a variety of memory cell embodiments. The MTJ cell essentially includes a pair of magnetic layers with an insulating layer sandwiched therebetween. One of the magnetic layers has a fixed magnetic vector and the other magnetic layer has a changeable (free) magnetic vector that is stable when either aligned with or opposed to the fixed magnetic vector. When the magnetic vectors are aligned, the resistance of the MTJ cell, i.e. the resistance to current flow between the magnetic layers, is a minimum, Rmin, and when the magnetic vectors are opposed, or misaligned, the resistance of the MTJ cell is a maximum, Rmax. The logic state of the MTJ cell is usually determined by a direct or indirect measurement of its electrical resistance.
Different read schemes have been proposed to determine the logic state of MTJ cells in an MRAM array. In the case of midpoint approaches, a read signal is applied to a given MTJ cell and an electrical property is measured and compared to a reference value in order to determine its logic state. For example, a sense amplifier can be used to measure the current passing through a cell in response to a voltage drop across the MTJ. The reference signal can be set as the midpoint of the current flowing through a reference bit in the low state and a reference bit in the high state.
Process variations, such as dielectric thickness or MTJ size, will cause the resistances Rmin and Rmax to vary within different cells of the array. Midpoint reference read techniques require high magneto-resistance (MR) in order to provide sufficient separation of the Rmin and Rmax distributions to reliably determine the logic state of the cell. Given the resistance and MR distributions measured in MRAM arrays, achieving MR values required for midpoint read approaches can be challenging.
Self-reference techniques have been developed to overcome the shortcomings of midpoint techniques and to provide a reliable way of determining the MTJ cell logic state without requiring high MR. In this case, the resistance of a MTJ cell is first sampled, then the MTJ cell is written to a known state, and finally the resistance of the cell is sampled again and compared to the first value. The comparison of the resistance of the cell before and after the write operation allows determining its logic state. This process is called a destructive read since the initial state of the MTJ cell is lost during the read operation.
Although self-reference techniques provide a better sensitivity than midpoint techniques, they have several disadvantages. The destructive read requires the MTJ cell to be written back to its initial state after the read operation is completed. The write operations involved during the read result in increased power consumption, longer cycle times, and additional stress of the tunnel junction eventually leading to reduced lifetime of the memory.
Accordingly, it is desirable to use an MTJ configuration in a midpoint reference MRAM without requiring a high MR. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.